Why not Java? It's more powerful, mostly faster, has superb tooling, high quality libraries for just about everything, tons of very experienced developers, etc.
Particularly look at the orange bars in the graph "Programming Languages" on [1] which is based on data from AngelList.
1: http://codingvc.com/which-technologies-do-startups-use-an-ex...
In the FinTech world most of the developers with deep domain knowledge will know Java/C++.
Of course it really depends on what the OPs startup is trying to do.
In fact even in my city (Cincinnti, OH) there are more Java developers than any other language, but they are mostly in corporate jobs.
So here still there are less Java developers working in startups compared to other languages.
Longer or harder than using Golang? Unlikely.
Golang has some advantages over Java like a simpler deployment model, less runtime overhead, etc but overall for a fairly standard web application it makes much more sense to pick Java over Golang.
> but before then it's premature optimisation in my book.
Then so is using Golang. You have answered your own original question :). Keep using Ruby.
Developer productivity is much higher with Golang.